Method and apparatus for multiple-input multiple-output operation

ABSTRACT

A wireless transmit/receive unit (WTRU) receives a downlink transmission from an eNodeB including a plurality of spatial layers transmitted to a plurality of WTRUs paired for multi-user MIMO. The WTRU decodes the downlink transmission based on a corresponding WTRU-specific reference signal. WTRU-specific reference signals are multiplexed using a frequency and code division multiplexing. WTRU-specific reference signals for different sub-group of antenna ports are multiplexed onto different subcarriers in a frequency domain and the WTRU-specific reference signals of the same sub-group of antennas ports are multiplied by an orthogonal cover code. The resource elements used for the WTRU-specific reference signals on antenna ports in one sub-group may be muted on antenna ports in another sub-group. A WTRU may transmit the uplink demodulation reference signal without precoding. The number of resource blocks allocated for the demodulation reference signal may be greater than the number of resource blocks allocated for the payload.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. provisional application No.61/522,934 filed Aug. 12, 2011, the contents of which is herebyincorporated by reference herein.

BACKGROUND

The closed-loop precoding scheme in Long Term Evolution Advanced (LTE-A)is extended to support configuration with up to 8 transmit antennas inthe downlink using a wireless transmit/receive unit (WTRU)-specificreference signal to improve data coverage. Since the WTRU-specificdemodulation reference signal (DM RS) is precoded in the same way as thedata, a non-codebook based precoding may be applied.

The orthogonality among the DM RS ports in the LTE Release 10 isachieved using a combination of code division multiplexing (CDM) andfrequency division multiplexing (FDM). FIG. 17 shows a conventionalWTRU-specific reference signal pattern for antenna ports 7, 8, 9, and 10for normal cyclic prefix. Total 12 resource elements (REs) are used perlayer for each subframe. Two CDM groups occupy different subcarriers ina frequency domain. In LTE Release 10, antenna ports 7, 8, 11, and 13belong to CDM group 1, and antenna ports 9, 10, 12, and 14 belong to CDMgroup 2. For rank-1 to rank-4, a length-2 orthogonal cover code (OCC) isapplied on two adjacent REs in each slot while for rank-5 to rank-8, alength-4 OCC is applied on two groups of consecutive REs in thesubframe.

A WTRU receives information regarding the assigned DM RS antennaport(s), the scrambling identity, and the number of layers throughdownlink control information, (e.g., a 3-bit field in DCI format 2C asspecified in Table 1). n_(SCID) is the scrambling identity for antennaports 7 and 8.

TABLE 1 One Codeword: Two Codewords: Codeword 0 enabled, Codeword 0enabled, Codeword 1 disabled Codeword 1 enabled Value Message ValueMessage

In LTE Release 10, for multi-user multiple-input multiple-output(MU-MIMO) operation, up to four layers can be simultaneously scheduledon the same frequency-time resources. This is accomplished throughmultiplexing of layers on two orthogonal DM RS antenna ports (antennaports 7 and 8) with two scrambling sequences, n_(SCID)={0,1} (the shadedrows in Table 1).

SUMMARY

A method and apparatus for multiple-input multiple-output (MIMO)transmissions are disclosed. A wireless transmit/receive unit (WTRU)receives a downlink transmission from an eNodeB including a plurality ofspatial layers transmitted to a plurality of WTRUs paired for multi-userMIMO. The WTRU decodes the downlink transmission based on aWTRU-specific reference signal. WTRU-specific reference signals for theplurality of WTRUs may be multiplexed into the downlink transmissionsuch that WTRU-specific reference signals for different sub-group ofantenna ports are multiplexed onto different subcarriers in a frequencydomain and the WTRU-specific reference signals of the same sub-group ofantennas ports are applied with an orthogonal cover code in a timedomain. The resource elements used for transmission of the WTRU-specificreference signals on antenna ports in one sub-group may be muted onantenna ports in another sub-group. The WTRU may decode the downlinktransmission assuming that no data is transmitted to the WTRU on themuted resource elements.

Four antenna ports with two scrambling sequences may be used to supportup to eight (8) spatial layers. A bandwidth assigned for the WTRU may bedifferent from a bandwidth assigned for a WTRU paired for multi-userMIMO. In this case, information regarding the largest bandwidth amongWTRUs paired for multi-user MIMO may be provided to the WTRU.

In another embodiment, a WTRU may transmit the uplink demodulationreference signal without precoding.

In another embodiment, the number of resource blocks allocated for theuplink demodulation reference signal may be greater than the number ofresource blocks allocated for the payload.

In another embodiment, the uplink demodulation reference signals from aplurality of WTRUs may be multiplexed in a frequency domain.

BRIEF DESCRIPTION OF THE DRAWINGS

A more detailed understanding may be had from the following description,given by way of example in conjunction with the accompanying drawingswherein:

FIG. 1A is a system diagram of an example communications system in whichone or more disclosed embodiments may be implemented;

FIG. 1B is a system diagram of an example wireless transmit/receive unit(WTRU) that may be used within the communications system illustrated inFIG. 1A;

FIG. 1C is a system diagram of an example radio access network and anexample core network that may be used within the communications systemillustrated in FIG. 1A;

FIG. 2 shows an example of splitting WTRUs into two code divisionmultiplexing (CDM) groups;

FIG. 3 shows an example for demodulation reference signal (DM RS) mutingfor MU-MIMO operation up to 8 layers;

FIG. 4 shows an example DM RS of two WTRUs with different number ofresources blocks multiplexed with a repetition factor (RPF) of 2;

FIG. 5 shows an example DM RS of three WTRUs with the same number ofresource blocks multiplexed with a RPF of 3;

FIGS. 6A and 6B show an example of employing the relative frequencyshifts to the DM RS;

FIG. 7A shows resource allocation of DM RS in MU-MIMO users inaccordance with conventional method;

FIG. 7B shows an example resource allocation of DM RS in MU-MIMO usersin accordance with one embodiment;

FIG. 8 shows an example resource allocation for the DM RS and thephysical uplink shared channel (PUSCH) payload in accordance with oneembodiment;

FIG. 9 shows a conventional uplink DM RS structure for a PUSCH;

FIG. 10 shows an example for the DM RS structure for a PUSCH defined asa combination of the legacy DM RS and the additional DM RS;

FIG. 11 shows an example uplink (UL) DM RS structure with the length-4OCC mapping;

FIG. 12 shows an example UL DM RS structure with interlaced allocationof the additional DM RS;

FIGS. 13A and 13B show two example DM RS patterns in accordance with oneembodiment;

FIGS. 14A and 14B show different patterns of DM RS with a differentshift in frequency domain, respectively;

FIG. 15 shows an example of length-4 OCC mapping for the DM RS;

FIG. 16 shows an example uplink control information (UCI) multiplexingaccording to their priorities; and

FIG. 17 shows a conventional WTRU-specific reference signal pattern forantenna ports 7, 8, 9, and 10 for normal cyclic prefix.

DETAILED DESCRIPTION

FIG. 1A is a diagram of an example communications system 100 in whichone or more disclosed embodiments may be implemented. The communicationssystem 100 may be a multiple access system that provides content, suchas voice, data, video, messaging, broadcast, etc., to multiple wirelessusers. The communications system 100 may enable multiple wireless usersto access such content through the sharing of system resources,including wireless bandwidth. For example, the communications systems100 may employ one or more channel access methods, such as code divisionmultiple access (CDMA), time division multiple access (TDMA), frequencydivision multiple access (FDMA), orthogonal FDMA (OFDMA), single-carrierFDMA (SC-FDMA), and the like.

As shown in FIG. 1A, the communications system 100 may include wirelesstransmit/receive units (WTRUs) 102 a, 102 b, 102 c, 102 d, a radioaccess network (RAN) 104, a core network 106, a public switchedtelephone network (PSTN) 108, the Internet 110, and other networks 112,though it will be appreciated that the disclosed embodiments contemplateany number of WTRUs, base stations, networks, and/or network elements.Each of the WTRUs 102 a, 102 b, 102 c, 102 d may be any type of deviceconfigured to operate and/or communicate in a wireless environment. Byway of example, the WTRUs 102 a, 102 b, 102 c, 102 d may be configuredto transmit and/or receive wireless signals and may include userequipment (UE), a mobile station, a fixed or mobile subscriber unit, apager, a cellular telephone, a personal digital assistant (PDA), asmartphone, a laptop, a netbook, a personal computer, a wireless sensor,consumer electronics, and the like.

The communications systems 100 may also include a base station 114 a anda base station 114 b. Each of the base stations 114 a, 114 b may be anytype of device configured to wirelessly interface with at least one ofthe WTRUs 102 a, 102 b, 102 c, 102 d to facilitate access to one or morecommunication networks, such as the core network 106, the Internet 110,and/or the networks 112. By way of example, the base stations 114 a, 114b may be a base transceiver station (BTS), a Node-B, an eNode B, a HomeNode B, a Home eNode B, a site controller, an access point (AP), awireless router, and the like. While the base stations 114 a, 114 b areeach depicted as a single element, it will be appreciated that the basestations 114 a, 114 b may include any number of interconnected basestations and/or network elements.

The base station 114 a may be part of the RAN 104, which may alsoinclude other base stations and/or network elements (not shown), such asa base station controller (BSC), a radio network controller (RNC), relaynodes, etc. The base station 114 a and/or the base station 114 b may beconfigured to transmit and/or receive wireless signals within aparticular geographic region, which may be referred to as a cell (notshown). The cell may further be divided into cell sectors. For example,the cell associated with the base station 114 a may be divided intothree sectors. Thus, in one embodiment, the base station 114 a mayinclude three transceivers, i.e., one for each sector of the cell. Inanother embodiment, the base station 114 a may employ multiple-inputmultiple output (MIMO) technology and, therefore, may utilize multipletransceivers for each sector of the cell.

The base stations 114 a, 114 b may communicate with one or more of theWTRUs 102 a, 102 b, 102 c, 102 d over an air interface 116, which may beany suitable wireless communication link (e.g., radio frequency (RF),microwave, infrared (IR), ultraviolet (UV), visible light, etc.). Theair interface 116 may be established using any suitable radio accesstechnology (RAT).

More specifically, as noted above, the communications system 100 may bea multiple access system and may employ one or more channel accessschemes, such as CDMA, TDMA, FDMA, OFDMA, SC-FDMA, and the like. Forexample, the base station 114 a in the RAN 104 and the WTRUs 102 a, 102b, 102 c may implement a radio technology such as Universal MobileTelecommunications System (UMTS) Terrestrial Radio Access (UTRA), whichmay establish the air interface 116 using wideband CDMA (WCDMA). WCDMAmay include communication protocols such as High-Speed Packet Access(HSPA) and/or Evolved HSPA (HSPA+). HSPA may include High-Speed DownlinkPacket Access (HSDPA) and/or High-Speed Uplink Packet Access (HSUPA).

In another embodiment, the base station 114 a and the WTRUs 102 a, 102b, 102 c may implement a radio technology such as Evolved UMTSTerrestrial Radio Access (E-UTRA), which may establish the air interface116 using Long Term Evolution (LTE) and/or LTE-Advanced (LTE-A).

In other embodiments, the base station 114 a and the WTRUs 102 a, 102 b,102 c may implement radio technologies such as IEEE 802.16 (i.e.,Worldwide Interoperability for Microwave Access (WiMAX)), CDMA2000,CDMA2000 1×, CDMA2000 EV-DO, Interim Standard 2000 (IS-2000), InterimStandard 95 (IS-95), Interim Standard 856 (IS-856), Global System forMobile communications (GSM), Enhanced Data rates for GSM Evolution(EDGE), GSM EDGE (GERAN), and the like.

The base station 114 b in FIG. 1A may be a wireless router, Home Node B,Home eNode B, or access point, for example, and may utilize any suitableRAT for facilitating wireless connectivity in a localized area, such asa place of business, a home, a vehicle, a campus, and the like. In oneembodiment, the base station 114 b and the WTRUs 102 c, 102 d mayimplement a radio technology such as IEEE 802.11 to establish a wirelesslocal area network (WLAN). In another embodiment, the base station 114 band the WTRUs 102 c, 102 d may implement a radio technology such as IEEE802.15 to establish a wireless personal area network (WPAN). In yetanother embodiment, the base station 114 b and the WTRUs 102 c, 102 dmay utilize a cellular-based RAT (e.g., WCDMA, CDMA2000, GSM, LTE,LTE-A, etc.) to establish a picocell or femtocell. As shown in FIG. 1A,the base station 114 b may have a direct connection to the Internet 110.Thus, the base station 114 b may not be required to access the Internet110 via the core network 106.

The RAN 104 may be in communication with the core network 106, which maybe any type of network configured to provide voice, data, applications,and/or voice over internet protocol (VoIP) services to one or more ofthe WTRUs 102 a, 102 b, 102 c, 102 d. For example, the core network 106may provide call control, billing services, mobile location-basedservices, pre-paid calling, Internet connectivity, video distribution,etc., and/or perform high-level security functions, such as userauthentication. Although not shown in FIG. 1A, it will be appreciatedthat the RAN 104 and/or the core network 106 may be in direct orindirect communication with other RANs that employ the same RAT as theRAN 104 or a different RAT. For example, in addition to being connectedto the RAN 104, which may be utilizing an E-UTRA radio technology, thecore network 106 may also be in communication with another RAN (notshown) employing a GSM radio technology.

The core network 106 may also serve as a gateway for the WTRUs 102 a,102 b, 102 c, 102 d to access the PSTN 108, the Internet 110, and/orother networks 112. The PSTN 108 may include circuit-switched telephonenetworks that provide plain old telephone service (POTS). The Internet110 may include a global system of interconnected computer networks anddevices that use common communication protocols, such as thetransmission control protocol (TCP), user datagram protocol (UDP) andthe internet protocol (IP) in the TCP/IP internet protocol suite. Thenetworks 112 may include wired or wireless communications networks ownedand/or operated by other service providers. For example, the networks112 may include another core network connected to one or more RANs,which may employ the same RAT as the RAN 104 or a different RAT.

Some or all of the WTRUs 102 a, 102 b, 102 c, 102 d in thecommunications system 100 may include multi-mode capabilities, i.e., theWTRUs 102 a, 102 b, 102 c, 102 d may include multiple transceivers forcommunicating with different wireless networks over different wirelesslinks. For example, the WTRU 102 c shown in FIG. 1A may be configured tocommunicate with the base station 114 a, which may employ acellular-based radio technology, and with the base station 114 b, whichmay employ an IEEE 802 radio technology.

FIG. 1B is a system diagram of an example WTRU 102. As shown in FIG. 1B,the WTRU 102 may include a processor 118, a transceiver 120, atransmit/receive element 122, a speaker/microphone 124, a keypad 126, adisplay/touchpad 128, non-removable memory 130, removable memory 132, apower source 134, a global positioning system (GPS) chipset 136, andother peripherals 138. It will be appreciated that the WTRU 102 mayinclude any sub-combination of the foregoing elements while remainingconsistent with an embodiment.

The processor 118 may be a general purpose processor, a special purposeprocessor, a conventional processor, a digital signal processor (DSP), aplurality of microprocessors, one or more microprocessors in associationwith a DSP core, a controller, a microcontroller, Application SpecificIntegrated Circuits (ASICs), Field Programmable Gate Array (FPGAs)circuits, any other type of integrated circuit (IC), a state machine,and the like. The processor 118 may perform signal coding, dataprocessing, power control, input/output processing, and/or any otherfunctionality that enables the WTRU 102 to operate in a wirelessenvironment. The processor 118 may be coupled to the transceiver 120,which may be coupled to the transmit/receive element 122. While FIG. 1Bdepicts the processor 118 and the transceiver 120 as separatecomponents, it will be appreciated that the processor 118 and thetransceiver 120 may be integrated together in an electronic package orchip.

The transmit/receive element 122 may be configured to transmit signalsto, or receive signals from, a base station (e.g., the base station 114a) over the air interface 116. For example, in one embodiment, thetransmit/receive element 122 may be an antenna configured to transmitand/or receive RF signals. In another embodiment, the transmit/receiveelement 122 may be an emitter/detector configured to transmit and/orreceive IR, ITV, or visible light signals, for example. In yet anotherembodiment, the transmit/receive element 122 may be configured totransmit and receive both RF and light signals. It will be appreciatedthat the transmit/receive element 122 may be configured to transmitand/or receive any combination of wireless signals.

In addition, although the transmit/receive element 122 is depicted inFIG. 1B as a single element, the WTRU 102 may include any number oftransmit/receive elements 122. More specifically, the WTRU 102 mayemploy MIMO technology. Thus, in one embodiment, the WTRU 102 mayinclude two or more transmit/receive elements 122 (e.g., multipleantennas) for transmitting and receiving wireless signals over the airinterface 116.

The transceiver 120 may be configured to modulate the signals that areto be transmitted by the transmit/receive element 122 and to demodulatethe signals that are received by the transmit/receive element 122. Asnoted above, the WTRU 102 may have multi-mode capabilities. Thus, thetransceiver 120 may include multiple transceivers for enabling the WTRU102 to communicate via multiple RATs, such as UTRA and IEEE 802.11, forexample.

The processor 118 of the WTRU 102 may be coupled to, and may receiveuser input data from, the speaker/microphone 124, the keypad 126, and/orthe display/touchpad 128 (e.g., a liquid crystal display (LCD) displayunit or organic light-emitting diode (OLED) display unit). The processor118 may also output user data to the speaker/microphone 124, the keypad126, and/or the display/touchpad 128. In addition, the processor 118 mayaccess information from, and store data in, any type of suitable memory,such as the non-removable memory 106 and/or the removable memory 132.The non-removable memory 106 may include random-access memory (RAM),read-only memory (ROM), a hard disk, or any other type of memory storagedevice. The removable memory 132 may include a subscriber identitymodule (SIM) card, a memory stick, a secure digital (SD) memory card,and the like. In other embodiments, the processor 118 may accessinformation from, and store data in, memory that is not physicallylocated on the WTRU 102, such as on a server or a home computer (notshown).

The processor 118 may receive power from the power source 134, and maybe configured to distribute and/or control the power to the othercomponents in the WTRU 102. The power source 134 may be any suitabledevice for powering the WTRU 102. For example, the power source 134 mayinclude one or more dry cell batteries (e.g., nickel-cadmium (NiCd),nickel-zinc (NiZn), nickel metal hydride (NiMH), lithium-ion (Li-ion),etc.), solar cells, fuel cells, and the like.

The processor 118 may also be coupled to the GPS chipset 136, which maybe configured to provide location information (e.g., longitude andlatitude) regarding the current location of the WTRU 102. In additionto, or in lieu of, the information from the GPS chipset 136, the WTRU102 may receive location information over the air interface 116 from abase station (e.g., base stations 114 a, 114 b) and/or determine itslocation based on the timing of the signals being received from two ormore nearby base stations. It will be appreciated that the WTRU 102 mayacquire location information by way of any suitablelocation-determination method while remaining consistent with anembodiment.

The processor 118 may further be coupled to other peripherals 138, whichmay include one or more software and/or hardware modules that provideadditional features, functionality and/or wired or wirelessconnectivity. For example, the peripherals 138 may include anaccelerometer, an e-compass, a satellite transceiver, a digital camera(for photographs or video), a universal serial bus (USB) port, avibration device, a television transceiver, a hands free headset, aBluetooth® module, a frequency modulated (FM) radio unit, a digitalmusic player, a media player, a video game player module, an Internetbrowser, and the like.

FIG. 1C is a system diagram of the RAN 104 and the core network 106according to an embodiment. As noted above, the RAN 104 may employ anE-UTRA radio technology to communicate with the WTRUs 102 a, 102 b, 102c over the air interface 116. The RAN 104 may also be in communicationwith the core network 106.

The RAN 104 may include eNode-Bs 140 a, 140 b, 140 c, though it will beappreciated that the RAN 104 may include any number of eNode-Bs whileremaining consistent with an embodiment. The eNode-Bs 140 a, 140 b, 140c may each include one or more transceivers for communicating with theWTRUs 102 a, 102 b, 102 c over the air interface 116. In one embodiment,the eNode-Bs 140 a, 140 b, 140 c may implement MIMO technology. Thus,the eNode-B 140 a, for example, may use multiple antennas to transmitwireless signals to, and receive wireless signals from, the WTRU 102 a.

Each of the eNode-Bs 140 a, 140 b, 140 c may be associated with aparticular cell (not shown) and may be configured to handle radioresource management decisions, handover decisions, scheduling of usersin the uplink and/or downlink, and the like. As shown in FIG. 1C, theeNode-Bs 140 a, 140 b, 140 c may communicate with one another over an X2interface.

The core network 106 shown in FIG. 1C may include a mobility managementgateway (MME) 142, a serving gateway 144, and a packet data network(PDN) gateway 146. While each of the foregoing elements are depicted aspart of the core network 106, it will be appreciated that any one ofthese elements may be owned and/or operated by an entity other than thecore network operator.

The MME 142 may be connected to each of the eNode-Bs 142 a, 142 b, 142 cin the RAN 104 via an S1 interface and may serve as a control node. Forexample, the MME 142 may be responsible for authenticating users of theWTRUs 102 a, 102 b, 102 c, bearer activation/deactivation, selecting aparticular serving gateway during an initial attach of the WTRUs 102 a,102 b, 102 c, and the like. The MME 142 may also provide a control planefunction for switching between the RAN 104 and other RANs (not shown)that employ other radio technologies, such as GSM or WCDMA.

The serving gateway 144 may be connected to each of the eNode Bs 140 a,140 b, 140 c in the RAN 104 via the S1 interface. The serving gateway144 may generally route and forward user data packets to/from the WTRUs102 a, 102 b, 102 c. The serving gateway 144 may also perform otherfunctions, such as anchoring user planes during inter-eNode B handovers,triggering paging when downlink data is available for the WTRUs 102 a,102 b, 102 c, managing and storing contexts of the WTRUs 102 a, 102 b,102 c, and the like.

The serving gateway 144 may also be connected to the PDN gateway 146,which may provide the WTRUs 102 a, 102 b, 102 c with access topacket-switched networks, such as the Internet 110, to facilitatecommunications between the WTRUs 102 a, 102 b, 102 c and IP-enableddevices.

The core network 106 may facilitate communications with other networks.For example, the core network 106 may provide the WTRUs 102 a, 102 b,102 c with access to circuit-switched networks, such as the PSTN 108, tofacilitate communications between the WTRUs 102 a, 102 b, 102 c andtraditional land-line communications devices. For example, the corenetwork 106 may include, or may communicate with, an IP gateway (e.g.,an IP multimedia subsystem (IMS) server) that serves as an interfacebetween the core network 106 and the PSTN 108. In addition, the corenetwork 106 may provide the WTRUs 102 a, 102 b, 102 c with access to thenetworks 112, which may include other wired or wireless networks thatare owned and/or operated by other service providers.

Embodiments for downlink DM RS enhancements for MU-MIMO are disclosedhereafter.

MIMO implementations such as those with a macro-node with low powerremote radio heads (RRH) may support more WTRUs within their respectivecoverage area. However, in case of MU-MIMO in LTE Release 10, which usesantenna ports 7 and 8 with two scrambling sequences, up to four layersmay be co-scheduled on the same frequency-time resources (i.e., resourceblocks). Out of these four layers two layers are fully orthogonal butthe other two layers are quasi-orthogonal due to the use of the twoscrambling sequences. This results in loss of orthogonality among theWTRUs which in turn adversely impacts system throughput.

In one embodiment, in order to improve orthogonality among the layersand increase the number of layers for MU-MIMO operation, a new set of DMRS ports may be defined for MU-MIMO operation. For example, the MU-MIMOoperation may be performed using DM RS antennas ports 7, 8, 9, and 10,(i.e., both CDM group 1 (antenna ports 7 and 8) and CDM group 2 (antennaports 9 and 10) may be used for MU-MIMO operation). With thisembodiment, up to eight layers may be simultaneously co-scheduled forMU-MIMO operation with two scrambling sequences.

The eNodeB scheduler (or any other network node) may split the WTRUs forMU-MIMO operation into multiple (e.g., two) groups based on theirspatial correlation, and assign each group of WTRUs to a different CDMgroup (i.e., the antenna ports that are multiplexed onto the samefrequency resource using OCCs). The eNodeB may inform the WTRUs in eachgroup their assigned DM RS CDM group either semi-statically ordynamically using layer 3, 2, or 1 signaling. FIG. 2 shows an example ofsplitting WTRUs into two CDM groups. In this example, WTRU 202 and WTRU204 are grouped together and assigned to CDM group 1, and WTRU 206 andWTRU 208 are grouped together and assigned to CDM group 2 based on theirspatial correlation.

A WTRU may be configured through a higher layer message or downlinkcontrol information (e.g., DCI Format 2C) regarding the assigned DM RSport(s). Table 2 shows an example for indicating the antenna port(s),the scrambling identity (n_(SCID)), and the number of layers for MU-MIMOoperations in CDM group 2 using 3 bits in DCI format 2C. The WTRUdecodes its downlink assignment and then identifies the assigned antennaports, the scrambling identity, and the number of layers using the 3-bitfield in its assignment as specified in Table 1 or Table 2.

TABLE 2 One Codeword: Two Codewords: Codeword 0 enabled, Codeword 0enabled, Codeword 1 disabled Codeword 1 enabled Value Message ValueMessage

In one embodiment, in order to improve orthogonality among co-scheduledWTRUs within a CDM group and facilitate channel estimation at the WTRUs,DM RS muting may be employed. FIG. 3 shows an example for DM RS mutingfor MU-MIMO operation up to 8 layers. As shown in FIG. 3, the resourceelements used for transmission of WTRU-specific reference signals onantenna ports in CDM group 1 may not be used for any transmissions onantenna ports in CDM group 2, and vice versa.

In this embodiment, the WTRU may assume that rate matching of data isperformed around the resource elements allocated for DM RS ports for theMU-MIMO (e.g., antenna ports 7, 8, 9, and 10), even though the WTRUemploys the assigned DM RS ports within one CDM group for channelestimation (e.g., either DM RS ports {7,8} or {9, 10}). The WTRU maydecode the downlink transmission assuming that no data is transmitted tothe WTRU on the muted resource elements. In this embodiment, in case anantenna port p is used (pε{7,8,9,10}), the WTRU may not assume that theother antenna ports in the set {7,8,9,10} are not associated withtransmission of PDSCH to another WTRU.

In LTE Release 10, in order to maintain orthogonality among co-scheduledWTRUs for MU-MIMO, the transmission bandwidth of those WTRUs paired forMU-MIMO operation is identical when the same DM RS port is used for bothWTRUs. In other words, unequal DL resource allocation for MU-MIMOoperation for more than two WTRUs is not possible in LTE Release 10.This reduces scheduling flexibility which in turn negatively impacts theachievable cell throughput.

In one embodiment, the MU-MIMO operation may be performed with unequalbandwidths, (i.e., resources of unequal bandwidth may be allocated tothe WTRUs paired for MU-MIMO). In order to enhance MU-MIMO operation,the largest transmission bandwidth among those WTRUs paired for MU-MIMOoperation may be signaled to all co-scheduled WTRUs within the same CDMgroup. The WTRU may use this information to generate the scramblingsequence applied on the DM RS, improve the interference measurementbased on the knowledge of the MU-MIMO co-scheduled interferer, calculatethe channel state information (CSI) feedback by taking into account theinterference caused by the MU-MIMO co-scheduled WTRU, enhance channelestimation using the DM RS information of the MU-MIMO co-scheduledinterferer, and the like.

N_(RB) ^(PDSCH) denotes the bandwidth in physical resource blocks of thecorresponding PDSCH transmission expressed in the number of RBs.Assuming K WTRUs are co-scheduled for MU-MIMO operation, the WTRU may beprovided, either through high layer signaling or downlink controlinformation (DCI), with an additional parameter N_(RB) ^(max,PDSCH) forthe largest bandwidth among the WTRUs paired for MU-MIMO, which may bederived by the following: Max{N_(RB,0) ^(PDSCH) N_(RB,1) ^(PDSCH) . . .N_(RB,K) ^(PDSCH)}. N_(RB,k) ^(PDSCH) denotes the total number ofallocated physical resource blocks to the k-th co-scheduled WTRU for theMU-MIMO operation.

Support for MU-MIMO operation with unequal bandwidth allocation requiresseparation of the DM RS of the WTRUs paired for MU-MIMO. In case theWTRUs are assigned the same DM RS port, in order to maintain theorthogonality among the co-scheduled layers, the scrambling function onthe WTRU-specific reference signal may be disabled for MU-MIMO operationwith unequal bandwidth allocation. Disabling the scrambling for theWTRU-specific reference signals may be done by higher layer or any othersignaling in a WTRU-specific manner.

Since MU-MIMO operation may not be used for cell edge WTRUs, theinter-cell interference due to the absence of scrambling may not be thelimiting factor for the performance of the co-scheduled MU-MIMO WTRUs.In case of severe inter-cell interference, the location of WTRU-specificreference signal resource elements may be shifted in a cell-specificmanner in the frequency or time domain. For example, in case the antennaports used for transmission of WTRU-specific reference signals to oneWTRU belong to the set S, where S={7,8,11,13} or S={9,10,12,14}, theresource elements for transmission of the WTRU-specific referencesignals may be shifted in the frequency domain according toν_(shift)=N_(ID) ^(cell) mod 4, where N_(ID) ^(cell) is the physicallayer cell identity.

Embodiments for enhancing the uplink DM RS are disclosed hereafter.

In LTE-A, the uplink DM RS sequences are derived from Zadoff-Chu (ZC) orcomputer-generated sequences. They have excellent auto-correlationproperties and near-constant amplitude in the time and frequency domain.Cyclic shifts of a DM RS sequence are mutually orthogonal, andwell-tailored for multiplexing. In LTE Release 8, cyclically shifted DMRS sequences are employed to support MU-MIMO when multiple WTRUs areco-scheduled with equal bandwidth allocation. Inter-cell interference isfurther randomized by employing sequence group hopping (SGH) within acell to change the base sequence for the DM RS every slot. Thisrandomizes the cross-correlation with the DM RS of the interferers fromneighboring cells and enables inter-cell interference mitigation.However, due to the cross-correlation effects, LTE Release 8 may notsupport MU-MIMO with flexible resource allocations wherein unequallengths of DM RS sequences may be multiplexed.

LTE-A Release 10 introduces a provision to facilitate flexiblescheduling for MU-MIMO. SGH may be disabled for some WTRUs in aWTRU-specific manner to make the DM RS sequences identical in the twoslots of a subframe and then an orthogonal cover code (OCC) of length-2may be applied across the DM RS symbols within a subframe. This schemeincreases the multiplexing capacity of the resources by a factor of 2.With SGH disabled, MU-MIMO WTRUs with different OCCs have orthogonalityon the DM RS regardless of their bandwidth allocations. However,disabling SGH increases inter-cell interference from a neighboring cellWTRU.

In one embodiment, the uplink DM RS of the WTRUs may be multiplexed in afrequency domain. If the WTRUs are assigned different frequency offsets,their DM RS's are orthogonal and may be multiplexed. The multiplexingmay be between intra-cell WTRUs and/or inter-cell WTRUs. If the constantamplitude zero autocorrelation (CAZAC) properties of the DM RS need tobe preserved, the DM RS sequence may be derived from the conventionalLTE Release 8/10 sequences (referred to as “base sequence” hereafter) byuniformly up-sampling an appropriate base sequence with a suitablerepetition factor (RPF). The LTE Release 8/10 bases sequences are allmultiples of 12.

FIG. 4 shows an example DM RS of two WTRUs with different number ofresources blocks (1 and 2 RBs in this example) multiplexed with a RPF of2. FIG. 5 shows an example DM RS of three WTRUs with the same number ofresource blocks (3 RBs in this example) multiplexed with a RPF of 3.

The WTRU may derive the base sequence length from its RB allocation andthe RPF. If the number of allocated resource blocks (N) is an integermultiple of the RPF, the WTRU may up-sample the base sequence of length12N/RPF to derive the DM RS sequence. For example, for RPF=2 and anallocation of 10 RBs, the WTRU may derive its DM RS sequence byup-sampling by a factor of 2 a base sequence of length 60 (=12×5/2).

If the number of allocated RBs (N) is not an integer multiple of theRPF, the DM RS may be derived by either cyclically extending ortruncating an up-sampled base sequence closest in length to 12N/RPF, orcyclically extending or truncating an up-sampled base sequence of lengthequal to the closest prime number. Currently, in LTE-A, the length ofthe base sequence for DM RS is a multiple of 12. This embodiment willrelax the requirement and allow more flexible allocations of DM RSlengths.

The frequency multiplexing scheme reduces the number of REs for the ULDM RS. This introduces a loss of power compared to LTE Release 10. Inorder to compensate, the WTRU may transmit the DM RS with more power.For example, for RPF=2, a WTRU may double the amplitude (power boost of6 dB) of its DM RS signal while keeping its payload at a nominal powerlevel.

The RPF on the DM RS and the WTRU frequency offsets may be fixed orconfigurable and may be based on the WTRU ID, the cell ID, or both. Thefrequency offset for a WTRU may change every slot or every subframe. Thefrequency offset pattern may be based on the WTRU ID, the cell ID, orboth.

The WTRUs may be flexibly assigned to operate in the frequency divisionmultiplexed mode in accordance with the embodiment above or the LTERelease 10 mode (i.e., DM RS without frequency division multiplexing),for example, through higher layer signaling or L1 signaling.

When SGH is disabled and WTRUs are multiplexed through the OCCs, aninter-cell WTRU with the same OCC may cause a high level ofinterference. The interference may be mitigated by introducing differentrelative frequency shifts (Δ) to the DM RS sequences of an interferingWTRU(s).

FIG. 6B shows an example of employing the relative frequency shifts(Δ=2) to the DM RS. In the LTE Release 10 scheme, as shown in FIG. 6A,the DM RS sequence in the second slot of a subframe is identical as SGHis disabled. An OCC of either [1 1] or [1 −1] may be applied to thesequences in two slots of the subframe.

In accordance with the embodiment, a relative shift of Δ is introducedin the DM RS sequences between the two slots in a subframe as shown inFIG. 6B. In FIG. 6B, the DM RS in the second slot of the subframe is thesame as that in the first slot except for a cyclic shift of 2.

If the relative shift (Δ) is different between the interfering WTRUs,the energy in interference may not be added coherently and as aconsequence the interference may be reduced. The relative shift (Δ) maychange across subframes and may be derived from the WTRU ID and/or thecell ID. The network may implement this embodiment to all WTRUs in acell or a subset of WTRUs.

In LTE Release 10, the DM RS OCCs are derived from the cyclic shiftfield (cyclicShift) in uplink-related DCI format. In one embodiment, theOCC may be derived as a function of time (e.g., the OCC may changeacross subframes). Alternatively or additionally, the OCC may be derivedbased on the WTRU ID and/or the cell ID. This will randomize theinterferers across the layers for the WTRU.

When MU-MIMO WTRUs have different resource allocations, their DM RSsequences are of different lengths and consequently have poorcross-correlation properties. In accordance with one embodiment, theMU-MIMO WTRUs may have different resource allocations of payload (e.g.,PUSCH+uplink control information (UCI)) but they may be assigned DM RSsequences of length corresponding to the larger resource allocation.Differences in the cyclic shifts of the sequences of the WTRUs mayensure orthogonality. In this case, the DM RS may be used for sounding.In accordance with this embodiment, SGH may not be disabled, therebyproviding more robustness to inter-cell interference. FIG. 7A showsresource allocation of DM RS in MU-MIMO users in accordance withconventional method. FIG. 7B shows an example resource allocation of DMRS in MU-MIMO users in accordance with this embodiment. In FIG. 7A, theDM RS 712, 714 for different WTRUs with different amount of resourceallocation are from different base sequences of different length and arenot orthogonal. In FIG. 7B, the DM RS 722, 724 for different WTRUs withdifferent amount of resource allocation are from the same base sequenceof the same length with different cyclic shifts and are orthogonal.

Embodiments for using DM RS for sounding are disclosed hereafter.

In LTE-A, the sounding resource signal (SRS) is used to enable channelsounding at the eNodeB. Channel sounding is performed to estimate aprecoding matrix indicator (PMI) and a rank, and to perform frequencyselective scheduling (FSS), etc. In LTE Release 8, when a WTRU isscheduled for sounding, it transmits the SRS in the last SC-FDMA symbolof a subframe using its assigned resources (periodicity, transmissioncomb, transmission bandwidth, cyclic shift, etc.). In LTE Release 8,which supports a single layer per WTRU and a single single-inputmultiple-output (SIMO) transmission mode, the DM RS may be used forchannel sounding in addition to channel estimation for demodulating thepayload. In other words, the DM RS may play the role of the SRS.

However, LTE Release 10 limits the role of DM RS in sounding. LTERelease 10 supports multi-layer transmission modes wherein a DM RSresource is applied to each layer, not per transmit antenna.Consequently, the DM RS can be used to sound the composite channel atthe receiver (pre-coded channel), not the individual channels (for everytransmit-receive antenna pair) whose knowledge is a pre-requisite formeaningful layer 1 feedback. Currently, the individual channels may notbe derived from the composite channel estimates.

In one embodiment, a non-precoded DM RS may be used on the UL so thatevery transmit-receive link may be estimated by the eNodeB. A DM RSresource may be allocated for every transmit antenna. When the number ofantennas is greater than the number of transmitted layers, more DM RSresources would be used than does in LTE Release 10. A non-precoded DMRS may be configured by semi-static signaling (e.g., RRC signaling) orby a dynamic indication (e.g., a modified DCI format 4).

Conventionally, the number of physical resource blocks (PRBs) allocatedfor the DM RS is the same as that allocated for the payload (e.g., PUSCHpayload). In one embodiment, if the DM RS-based sounding is performed,the DM RS allocation may be independent of the allocation for thepayload. FIG. 8 shows an example resource allocation for the DM RS andthe PUSCH payload in accordance with one embodiment. In FIG. 8, the DMRS (the middle SC-FDMA symbol 802) has a larger bandwidth than thepayload (the remaining SC-FDMA symbols 804). The resource allocation forthe DM RS may be indicated via higher layer signaling (e.g., RRC) orlayer 1 or 2 signaling when the WTRU is scheduled for sounding. In aspecial case, a DM RS may be scheduled for sounding without PUSCHallocation.

The number of required resources may increase with non-precoded DM RS.In one embodiment, more DM RS sequences may be configured for a cell byassigning an additional cell ID(s) (N_(ID) ^(cell)) to the cell. N_(ID)^(cell) is a physical layer cell identity. For example, the network mayassign an additional set of DM RS sequences to the cells by providing anoffset δ to the N_(ID) ^(cell). Each cell may have DM RS resourcescorresponding to both its N_(ID) ^(cell) and N_(ID) ^(cell)+δ. The DM RSsequences generated from the two cell IDs may be orthogonalized by usingOCCs across the two slots within the subframe with SGH disabled.

The above embodiment may be used to interleave the DM RS resources inboth SU-MIMO and MU-MIMO when sounding with DM RS.

The WTRU may use time division multiplexing of its DM RS resource acrossthe time slots within a subframe. For example, the WTRU may allocate aresource to a first antenna in slot 1 of a subframe and no resource to asecond antenna in the same slot, while the WTRU may allocate a resourceto the second antenna in slot 2 and no resource to the first antenna inslot 2.

In another embodiment, precoded DM RS may be used for sounding and theindividual channels may be computed from the composite pre-codedchannels at the receiver (e.g., eNodeB). If different PMIs are used insuccessive transmissions within the coherence time of the channel, thereceiver may compute the individual channels for sounding from itsestimates of the composite channels. A WTRU may be configured totransmit different PMIs successively for sounding purposes. The sequenceof PMIs used by the WTRU may be configured by the higher layers or itmay be derived from the WTRU ID, the cell ID, or the like. In TDD, thechannels may be estimated based on reciprocity. In FDD, statistics suchas channel correlation on the DL may be used with reciprocity on the UL.

FIG. 9 shows a conventional uplink DM RS structure for a PUSCH. The DMRS allocation is TDM-based with a cyclic shift. Due to the TDM-based DMRS structure, the channel estimation accuracy may be degraded as aDoppler frequency becomes higher and/or a modulation order becomeshigher. In addition, the DM RS SC-FDMA symbol is located in the middleof the slot, thus requiring extrapolation to obtain channel statusinformation for the SC-FDMA symbols in the subframe boundary. This maylead demodulation performance degradation in high mobility case andhigher modulation order.

Table 3 shows cyclic shift allocation and OCC mapping for uplink DM RS.In this case, up to four layers may be multiplexed with a differentcyclic shift and an OCC and this may help to keep the orthogonalitybetween layers or multiple users.

TABLE 3 Sig- nal- ing Cyclic Shift(α) OCC(w) bit λ = 0 λ = 1 λ = 2 λ = 3λ = 0 λ = 1 λ = 2 λ = 3 000 0 6 9 3 [+1 +1] [+1 +1] [+1 −1] [+1 −1] 0016 0 3 9 [+1 −1] [+1 −1] [+1 +1] [+1 +1] 010 3 9 6 0 [+1 −1] [+1 −1] [+1+1] [+1 +1] 011 4 10 7 1 [+1 +1] [+1 +1] [+1 +1] [+1 +1] 100 2 8 5 11[+1 +1] [+1 +1] [+1 +1] [+1 +1] 101 8 2 11 5 [+1 −1] [+1 −1] [+1 −1] [+1−1] 110 10 4 1 7 [+1 −1] [+1 −1] [+1 −1] [+1 −1] 111 9 3 0 6 [+1 +1] [+1+1] [+1 −1] [+1 −1]

In order to keep the orthogonality with legacy WTRUs (i.e., LTE Releases8, 9, and 10) for multi-user MIMO, the DM RS pattern and multiplexingmay be maintained as in the previous LTE Releases. Given thisrestriction, an additional DM RS may be added on top of the legacy DM RSin order to improve the channel estimation accuracy, (e.g., under thehigh Doppler frequency). FIG. 10 shows an example for the DM RSstructure for a PUSCH which is defined as a combination of the legacy DMRS 1002 and the additional DM RS 1004. The same cyclic shifts and OCCsmay be used for the additional DM RS 1004.

In another embodiment, a length-4 OCC may be used over the legacy DM RS1002 and the additional DM RS 1004 instead of a length-2 OCC to increasethe number of orthogonal reference signals for better support ofmulti-user MIMO and uplink cooperative multipoint transmission (CoMP).Although the length-4 OCC is used, the backward compatibility may bemaintained as far as the same OCC is used for legacy DM RS 1002. FIG. 11shows an example UL DM RS structure with the length-4 OCC mapping. Sincethe length-2 OCC is a subset of the length-4 OCC, the subset is used forthe legacy DM RS to keep the backward compatibility.

Table 4 shows an example length-4 OCC mapping while keeping the legacyDM-RS length-2 OCC mapping as in the previous LTE Releases. Since theadditional DM RS 1004 is not orthogonal with the legacy WTRU whenco-scheduled in the same time and frequency resources, the length-4 OCCmay not be used with the legacy WTRU for multi-user MIMO transmission.

TABLE 4 Signaling OCC(w) for legacy DM RS OCC(w) for additional DM RSbit λ = 0 λ = 1 λ = 2 λ = 3 λ = 0 λ = 1 λ = 2 λ = 3 000 [+1 +1] [+1 +1][+1 −1] [+1 −1] [+1 +1] [−1 −1] [+1 −1] [−1 +1] 001 [+1 −1] [+1 −1] [+1+1] [+1 +1] [+1 −1] [−1 +1] [+1 +1] [−1 −1] 010 [+1 −1] [+1 −1] [+1 +1][+1 +1] [+1 −1] [−1 +1] [+1 +1] [−1 −1] 011 [+1 +1] [+1 +1] [+1 +1] [+1+1] [+1 +1] [−1 −1] [+1 +1] [−1 −1] 100 [+1 +1] [+1 +1] [+1 +1] [+1 +1][+1 +1] [−1 −1] [+1 +1] [−1 −1] 101 [+1 −1] [+1 −1] [+1 −1] [+1 −1] [+1−1] [−1 +1] [+1 −1] [−1 +1] 110 [+1 −1] [+1 −1] [+1 −1] [+1 −1] [+1 −1][−1 +1] [+1 −1] [−1 +1] 111 [+1 +1] [+1 +1] [+1 −1] [+1 −1] [+1 +1] [−1−1] [+1 −1] [−1 +1]

Since the legacy DM RS structure is kept, the legacy WTRU performancefor multi-user MIMO may be kept as well. On the other hand, a WTRU usingthe DM RS pattern of FIG. 10 may have different channel estimationaccuracy according to the DM RS type. For instance, the channelestimation from the legacy DM RS may be better than that from theadditional DM RS since the additional DM RS is not orthogonal with thePUSCH symbol for other co-scheduled WTRU. In this case, the eNodeB mayallocate higher power for the additional DM RS and the power differencebetween the legacy DM RS and the additional DM RS may be informed to aWTRU, for example, by higher layer signaling.

Due to the introduction of the additional DM RS, the reference signaloverhead becomes doubled as compared to the previous LTE Releases. Thismay lead to WTRU throughput performance loss. In order to lower thereference signal overhead, interlaced allocation for the additional DMRS may be used as shown in FIG. 12.

It should be noted that even though FIGS. 10-12 show that the additionalDM RS is located in the first SC-FDMA symbol in each slot, the timelocation of the additional DM RS may be changed to other SC-FDMA symbolin each slot (e.g., the sixth or seventh SC-FDMA symbol). In addition,the additional DM RS position may vary in each slot.

In another embodiment, the DM RS pattern may designed differently. FIGS.13A and 13B show two example DM RS patterns in accordance with oneembodiment. As shown in FIGS. 13A and 13B as an example, in order tokeep the same DM RS overhead as the legacy DM RS pattern, the pattern ofDM RS 1302 interlaced in frequency domain may be used for 4 SC-FDMAsymbols.

FIGS. 14A and 14B show different patterns of DM RS 1402 with a differentshift (n-shift=0 or 1 in this example) in frequency domain,respectively. The data REs in the SC-FDMA symbols containing the DM RS1402 may be muted as shown in FIGS. 14A and 14B. The muting may be usedas a default or configured by higher layer signaling. By using then-shift with muting, the DM RS capacity may be increased (e.g., doubled)so that a larger number of WTRUs may be co-scheduled with orthogonal DMRS. In addition, orthogonality between multiple WTRUs located indifferent cell may be supported.

Since the DM RS is allocated with an interlaced pattern in an SC-FDMAsymbol, the available number of cyclic shift of the base sequence of theDM RS may be reduced under the same channel delay spread. The number oforthogonal DM RS ports may be increased by using length-4 OCC mapping.FIG. 15 shows an example of length-4 OCC mapping for the DM RS 1502. Thelayer may be indicated by cyclic shift in combination with OCC.

The DM RS resource may be allocated dynamically via a PDCCH. Threeparameters (cyclic shift, OCC, and n-shift) may be used to allocate theDM RS resources. Table 5 shows an example of the DMRS resource mapping.

TABLE 5 Signaling Cyclic shift and n-shift (α, μ) OCC bit λ = 0 λ = 1 λ= 2 λ = 3 λ = 0 λ = 1 λ = 2 Λ = 3 000 (0, 0) (6, 0) (9, 1) (3, 1) [+1 +1+1 +1] [+1 +1 +1 +1] [+1 +1 +1 +1] [+1 +1 +1 +1] 001 (6, 0) (0, 0)(3, 1) (9, 1) [+1 +1 −1 −1] [+1 +1 −1 −1] [+1 +1 −1 −1] [+1 +1 −1 −1]010 (3, 0) (9, 0) (6, 1) (0, 1) [+1 −1 +1 −1] [+1 −1 +1 −1] [+1 −1 +1−1] [+1 −1 +1 −1] 011 (4, 0) (10, 0) (7, 1) (1, 1) [+1 −1 −1 +1] [+1 −1−1 +1] [+1 −1 −1 +1] [+1 −1 −1 +1] 100 (2, 0) (8, 0) (5, 1) (11, 1)  [+1+1 +1 +1] [+1 +1 +1 +1] [+1 +1 +1 +1] [+1 +1 +1 +1] 101 (8, 0) (2, 0)(11, 1)  (5, 1) [+1 +1 −1 −1] [+1 +1 −1 −1] [+1 +1 −1 −1] [+1 +1 −1 −1]110 (10, 0)  (4, 0) (1, 1) (7, 1) [+1 −1 +1 −1] [+1 −1 +1 −1] [+1 −1 +1−1] [+1 −1 +1 −1] 111 (9, 0) (3, 0) (0, 1) (6, 1) [+1 −1 −1 +1] [+1 −1−1 +1] [+1 −1 −1 +1] [+1 −1 −1 +1]

Among the three parameters for the DM RS allocation, a subset of thethree parameters may be used in the system. Alternatively, a subset ofthe parameters (e.g., cyclic shift and OCC) may be indicated by a PDCCHand other parameters (e.g., n-shift) may be indicated by higher layersignaling.

If a WTRU needs to transmit a PUCCH and a PUSCH in the same subframe,uplink control information (UCI) on the PUCCH may be piggybacked on thePUSCH in order not to increase cubic metric (CM) for the SC-FDMAtransmission. The UCI includes hybrid automatic repeat request (HARQ)positive acknowledgement/negative acknowledgement (ACK/NACK), channelquality indicator (CQI), precoding matrix indicator (PMI), and rankindicator (RI). Among the UCIs, the HARQ ACK/NACK may have the highestpriority so that it may be multiplexed in the time/frequency resourcesthat may provide higher channel estimation accuracy. The RI may have asecond highest priority since the misdetection of RI may leadmisinterpretation of CQI/PMI information. The PMI/CQI may be multiplexwith the same manner as in previous releases. The UCIs may bemultiplexed according to their priorities. FIG. 16 shows an example UCImultiplexing according to their priorities.

Although features and elements are described above in particularcombinations, one of ordinary skill in the art will appreciate that eachfeature or element can be used alone or in any combination with theother features and elements. In addition, the methods described hereinmay be implemented in a computer program, software, or firmwareincorporated in a computer-readable medium for execution by a computeror processor. Examples of computer-readable media include electronicsignals (transmitted over wired or wireless connections) andcomputer-readable storage media. Examples of computer-readable storagemedia include, but are not limited to, a read only memory (ROM), arandom access memory (RAM), a register, cache memory, semiconductormemory devices, magnetic media such as internal hard disks and removabledisks, magneto-optical media, and optical media such as CD-ROM disks,and digital versatile disks (DVDs). A processor in association withsoftware may be used to implement a radio frequency transceiver for usein a WTRU, UE, terminal, base station, RNC, or any host computer.

The invention claimed is:
 1. A method for receiving multiple-input multiple-output (MIMO) transmissions, the method comprising: a wireless transmit/receive unit (WTRU) receiving at least one of a plurality of downlink transmissions from an eNodeB on at least one of a plurality of spatial layers, wherein the plurality of spatial layers are transmitted to a plurality of WTRUs for multi-user MIMO, and wherein the at least one spatial layer is used for reception by the WTRU; the WTRU receiving at least one WTRU-specific reference signal corresponding to the at least one downlink transmission; and the WTRU decoding the at least one downlink transmission based on the at least one corresponding WTRU-specific reference signal for the WTRU, wherein resource elements are allocated to resource blocks, and the resource blocks include WTRU-specific reference signals for the plurality of WTRUs and the downlink transmission such that WTRU-specific reference signals for different sub-groups of antenna ports are multiplexed onto different subcarriers in a frequency domain and the WTRU-specific reference signals of the same sub-group of antennas ports are each applied with a different orthogonal cover code in a time domain, wherein each sub-group of antenna ports includes at least two antenna ports, and wherein resource elements used for the downlink transmission are rate matched around resource elements allocated for the at least one WTRU-specific reference signal for the WTRU on at least one antenna port in one sub-group and are rate matched around resource elements allocated for at least one WTRU-specific reference signal for at least another WTRU on at least one antenna port in another sub-group, wherein the resource elements allocated for the at least one WTRU-specific reference signal for at least another WTRU are muted.
 2. The method of claim 1 wherein the WTRU decodes the downlink transmission assuming that no data is transmitted to the WTRU on the muted resource elements.
 3. The method of claim 1 wherein four antenna ports are used to support up to eight (8) layers.
 4. The method of claim 1 wherein the WTRUs are paired for multi-user MIMO based on spatial correlation among the WTRUs.
 5. The method of claim 1 wherein a bandwidth assigned for the WTRU is different from a bandwidth assigned for another WTRU for multi-user MIMO.
 6. The method of claim 5 wherein the WTRU receives information regarding a largest bandwidth among the plurality of WTRUs for multi-user MIMO.
 7. The method of claim 6 wherein the WTRU does not apply a scrambling sequence to the WTRU-specific reference signal on a condition that a bandwidth assigned to another WTRU for multi-user MIMO is different.
 8. The method of claim 1 wherein the downlink transmission is received on two spatial layers used for reception by the WTRU.
 9. A wireless transmit/receive unit (WTRU) comprising: a transceiver configured to receive at least one of a plurality of downlink transmissions from an eNodeB on at least one of a plurality of spatial layers, wherein the plurality of spatial layers are transmitted to a plurality of WTRUs for multi-user MIMO, and wherein the at least one spatial layer is used for reception by the WTRU; the transceiver configured to receive at least one WTRU-specific reference signal corresponding to the at least one downlink transmission; and a processor operatively coupled to the transceiver, the processor configured to decode the at least one downlink transmission based on the at least one corresponding WTRU-specific reference signal for the WTRU, wherein resource elements are allocated to resource blocks, and the resource blocks include WTRU-specific reference signals for the plurality of WTRUs and the downlink transmission such that WTRU-specific reference signals for different sub-groups of antenna ports are multiplexed onto different subcarriers in a frequency domain and the WTRU-specific reference signals of the same sub-group of antennas ports are each applied with a different orthogonal cover code in a time domain, wherein resource elements used for the downlink transmission are rate matched around resource elements allocated for the at least one WTRU-specific reference signal for the WTRU on at least one antenna port in one sub-group and are rate matched around resource elements allocated for at least one WTRU-specific reference signal for at least another WTRU on at least one antenna port in another sub-group, wherein the resource elements allocated for the at least one WTRU-specific reference signal for at least another WTRU are muted.
 10. The WTRU of claim 9 wherein the processor is configured to decode the downlink transmission assuming that no data is transmitted to the WTRU on the muted resource elements.
 11. The WTRU of claim 9 wherein four antenna ports are used to support up to eight (8) spatial layers.
 12. The WTRU of claim 9 wherein the WTRUs are paired for multi-user MIMO based on spatial correlation among the WTRUs.
 13. The WTRU of claim 9 wherein a bandwidth assigned for the WTRU is different from a bandwidth assigned for another WTRU for multi-user MIMO.
 14. The WTRU of claim 13 wherein the processor and the transceiver are configured to receive information regarding a largest bandwidth among the plurality of WTRUs or multi-user MIMO.
 15. The WTRU of claim 14 wherein the processor is configured to not apply a scrambling sequence to the WTRU-specific reference signal on a condition that a bandwidth assigned to another WTRU for multi-user MIMO is different.
 16. The WTRU of claim 9 wherein the downlink transmission is received on two spatial layers used for reception by the WTRU. 